Rupture of non-occlusive cerebrovascular lesions, such as intracranial saccular aneurysms or arterio-venous fistulae, are a major cause of stroke. Rupture of an aneurysm causes subarachnoid hemorrhage in which blood from a ruptured vessel spreads over the surface of the brain. About 2.5% of the United States population (4 million Americans) have an unruptured aneurysm. About 100,000 of these people suffer a subarachnoid hemorrhage. The disease is devastating, often affecting healthy people in their 40's and 50's, with about half of the rupture victims succumbing within a month, and with half of the survivors becoming seriously disabled as a result of the initial hemorrhage or of a delayed complication.
Neurovascular arteries are generally quite small, having diameters ranging from 2.0 to 4.0 mm in the Circle of Willis, 2.5 to 4.5 mm in the cavernous segment of the internal carotid artery, 1.5 to 3.0 mm in vessels of the distal anterior circulation, and 2.0 to 4.0 mm in the posterior circulation. The incidence of aneurysm varies with the location, with 55% occurring in the Circle of Willis, 30% in the internal carotid, 10% in the distal anterior circulation, and 5% in the posterior circulation.
Screening for these lesions and preventing rupture will lead to better clinical outcomes and lower costs. Non-invasive treatments for ruptured and unruptured lesions are preferred over surgical interventions due to lower costs, lower mortality and morbidity, and patient preference. An attractive treatment for ruptured and unruptured aneurysms is the placement of a stent within the lumen to prevent rupture or re-rupture of the lesion.
Stents formed of a helical coil or ribbon of shape-memory alloy material are known in the art. In general, such stents are formed to a desired expanded shape and size for vascular use above the transition temperature of the material. The stent is then cooled below its transition temperature and reshaped to a smaller-diameter coil suitable for catheter administration. After the stent in its contracted, smaller-diameter shape is delivered to the target site, e.g., via catheter, it is warmed by the body to above its transition temperature, causing the stent to assume its original expanded shape and size, typically a shape and size that anchors the stent against the walls of the vessels at the vascular site. Stents of this type are disclosed for example, in U.S. Pat. Nos. 4,512,338, 4,503,569, 4,553,545, 4,795,485, 4,820,298, 5,067,957, 5,551,954, 5,562,641, and 5,824,053. Also known in the art are graft-type stents designed for treating aneurysms, typically at relatively large-vessel sites, e.g., with vessel lumen sizes between about 15 and 30 mm. U.S. Pat. No. 4,512,338 is exemplary.
Stents such as disclosed heretofore have one or more of the following limitations, for purposes of the present invention:                (i) they are not capable of being advanced to a target site, such as a neurovascular site, that is accessible only along a tortuous path by a small-diameter catheter;        (ii) they may cause vessel injury due to rapid expansion at the target site;        (iii) they are not suitable for treating aneurysms in the absence of a special graft, sleeve or webbing;        (iv) they may cause thrombosis (clotting) of small vessels with low flow such as neurovascular vessels.        
It would therefore be desirable to provide a stent that overcomes these limitations, and which is suitable, in one embodiment, for use in treating neuroaneurysms.